1. Technical Field
The disclosure relates to electric double-layer capacitors. More particularly, the disclosure relates to electrolyte used in the electric double-layer capacitors.
2. Description of Related Art
Electric double-layer capacitors (EDLCs) are also known as supercapacitors. As its name suggests, the supercapacitor stores more energy per unit weight than the conventional capacitor. Furthermore, the electric double-layer capacitor exhibits better power rating than conventional rechargeable batteries during electric output. With the advantages such as long shelf life, good cycling characteristics, and wide operating temperature range, the electric double-layer capacitors have attracted attentions for their applications in energy storage and output.
Briefly, an electric double-layer capacitor includes a pair of polarizable electrodes and a separator interposed therebetween, wherein the electrodes and the separator are immersed in an electrolyte. When a potential difference is applied across the electrodes, cations in the electrolyte move toward the negative electrode whereas the anions in the electrolyte move toward the positive electrode and thereby forms an ionic current within the electric double-layer capacitor. When the positively charged- and negatively charged-ions arrive at their respective electrode, the charges would be adhered to the interface between the electrolyte and the respective electrode, whereas the electric field applied on the electrodes may keep the ions thereto and thereby may store the electric energy in the electric double-layer capacitor.
Up to now, how to decrease the internal resistance of an electric double-layer capacitor to improve its charging and discharging rates is still a hot research topic. Most of the efforts are focused on the materials of the polarizable electrodes. Many materials have been used to make the polarizable electrodes. For example, some various carbon materials, such as activated charcoal, graphene, and carbon nanotubes, some polymers having redox behavior and high surface area, and metal oxide have been used to make the polarizable electrodes. However, the internal resistance of the electric double-layer capacitors is not only affected by the internal resistance of the electrodes, but also the internal resistance of the electrolyte. But so far, not so many efforts are devoted to the electrolyte.